1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to a data forwarding apparatus and method for reducing a HandOver (HO) interruption time in a wireless communication system.
2. Description of the Related Art
Many wireless communication technologies have been proposed as candidates to support high speed mobile communication. Among these, the Orthogonal Frequency Division Multiplexing (OFDM) technology is now recognized as a leading next-generation wireless communication technology. In the future, it is expected that the OFDM technology will be used in most wireless communication technologies. At present, even the Institute Electrical and Electronics Engineers (IEEE) 802.16 Wireless Metropolitan Area Network (WMAN), referred to as a 3.5-Generation (3.5G) technology, has adopted the OFDM technology.
The OFDM scheme transmits data using a multi-carrier. Namely, the OFDM scheme is a type of Multi Carrier Modulation (MCM) scheme of parallel-converting symbol streams input in series and modulating each of the symbol streams into a plurality of sub-carriers having cross orthogonality (i.e., a plurality of sub-channels) for transmission.
In a cellular wireless communication system, there occurs a phenomenon in which a communication between a Mobile Station (MS) and a Base Station (BS) fails because a channel state deteriorates due to, for example, geographical conditions within a cell, a distance between the MS and the BS, a movement of the MS, and the like. For example, a propagation shadow area is formed even in a service area of the BS by a sealed building such as an office or a house. If the MS is positioned in the propagation shadow area, the BS may fail to reliably perform communication with the MS because of a poor channel state between the BS and the MS.
Thus, the wireless communication system may include a femto cell to provide a high speed data service while addressing a service problem in the propagation shadow area. A Femto cell is a small cell area formed by a compact BS, which is installed indoors such as in an office, a house, etc. The femto cell has access to a mobile communication core network through a broadband network. The compact BS, which is a small output BS and may be installed, for example, by a home owner or an MS user in an office, is referred to as a micro BS, a self configurable BS, an indoor BS, a home BS, a femto BS, etc. In the following description, the compact BS is referred to as the femto BS.
On the other hand, in order to guarantee the mobility of an MS, a wireless communication system including only a macro cell allows the MS to perform an HO procedure when leaving one macro cell and entering another macro cell. Thus, the MS performs the HO procedure by being in synchronization with a target BS and sending a HO confirmation message to the target BS. After that, the MS, which was serviced by a previous BS, establishes service with the target BS. At that time, data packets of the MS existing in a queue of the previous BS are transmitted to the target BS through a backbone network (i.e., a core network). This operation is referred to as ‘data forwarding’.
Handover interruption time is a performance indicator of great significance to a HO procedure. When performing the HO, an MS releases a connection with a previous BS and attempts to establish a connection with a target BS. Here, the HO interruption time can be defined as a period of time ranging from the moment of releasing the connection with the previous BS to a time of establishing the connection with the target BS and receiving new data packets from the target BS. The HO interruption time is an important performance indicator because it assists in determining if the continuity of service is maintained, i.e., a seamless service is possible. The HO interruption time is affected by a time of performing an HO procedure with the target BS. However, it is further affected by the period of time for receiving the new data packets from the target BS. That is, the HO interruption time is determined depending on when data packets of an MS, which are first transmitted from the previous BS to the target BS after the end of the HO procedure with the target BS, are transmitted to the MS.
In a case in which an MS exits a macro cell area and enters a femto cell area in a wireless communication system where a macro cell and a femto cell coexist, an HO from the macro cell to the femto cell is performed. At this time, if the existing HO technology between macro cells is applied to the HO from the macro cell to the femto cell, a macro BS attempts data forwarding through a backbone network. The femto cell maintains connectivity with the backbone network through an Internet Service Provider (ISP) network. By using the connection through the ISP, data packets are transmitted through the macro BS, a macro Mobility Management Entity (MME), a femto MME, an ISP network, and the femto BS. Based on this path, data traffic introduced into the femto BS experiences a latency effect caused by the ISP network. The latency due to the ISP network is different according to node distance. However, in IEEE 802.16m Evaluation Methodology Document (EMD), the latency is shown by the Laplacian distribution of an average of 80 ms. Because the latency time in the ISP network is long, data packets transmitted by the macro BS reach the femto BS late. Thus, even after performing the HO to the femto BS, an MS cannot receive data packets through data forwarding until waiting for a predetermined time. This results in an increase of the HO interruption time, thus causing a problem in which an MS receiving a Voice over Internet Protocol (VoIP) or video streaming service suffers from a service cut-off phenomenon.
A fundamental cause of the increase of the HO interruption time is that the data forwarding by the macro BS goes through the ISP network. Basically, the time taken to go through the ISP network is an average of 80 ms. Furthermore, the ISP network has difficulty in guaranteeing a Quality of Service (QoS) of data packets. Because of this, latency can further increase according to traffic loading. And, because of a sudden increase of Internet traffic indoors where the femto BS is installed, there occurs a bottleneck phenomenon in the line supplying the femto BS indoors, thus causing an even greater increase of latency in transmitting data packets.